1. Field of the Invention
The present invention relates to receiver circuits. Particularly, the present invention is directed to improving the performance of a receiver through a combination of a transmit/receive (T/R) switch protecting the receiver electronics with a Low-Noise Amplifier (LNA).
2. Description of Related Art
Generally, communications systems transfer information from a source to a destination using a combination of a transmitter and a receiver. Typically, the transmitter includes a transducer and a transmission element which together convert an electrical signal into an electromagnetic signal. The electromagnetic signal then propagates through a transmission medium to the receiver, which converts the signal with the help of another transducer into a desired form for a use by an end user. The transmission medium may be any of a variety of suitable devices, including copper cable, optical fiber or even air.
The present invention relates particularly to receivers for duplexed transducers, such as those typically present in ultrasound systems. In an ultrasound system, for example, a piezo-electric transducer converts an electronic signal to a sound wave. The same piezo-electric transducer may also perform the converse transformation such that a received sound wave is converted into an electrical signal. The present invention relates to systems where transducers are used for both the generation and the detection of ultrasound waves. Scanning along a line radiating from a transducer, as shown in FIG. 1, is performed as follows.
First a transmitter, in the ultrasound system creates a high-voltage pulse. The high-voltage pulse is applied to the transducer during a short transmission time interval to create an ultrasound wave. The ultrasound wave is reflected by an interface between regions of different acoustic impedance. For example, in medical imaging, the interface between the liver and surrounding tissues would reflect the ultrasound pulse. The reflected ultrasound pulse is absorbed during a reception time interval by the transducer, which creates a low-voltage pulse as a result. A receiver in the ultrasound system amplifies and processes the received low-voltage pulse.
A low-noise amplifier (LNA) is an electronic circuit of the receiver that amplifies the low-voltage pulse while minimizing the amount of electronic noise added to the low-voltage pulse. An important characteristic of this LNA is its input impedance. The input impedance of the LNA will present a load to the transducer. A very crude model of the transducer is a transformer operating between different domains as illustrated in FIG. 2. For ultrasound imaging system, for example, the two domains are electric and acoustic. In receiving mode, the acoustic signal stimulating the transducer can be modeled in the electrical domain as a voltage source. To maximize power conversion between domains by the transducer, it is necessary to load the transducer with an impedance equal to the impedance associated with the voltage source on the acoustic side of the transformer, as seen from the electric side. The LNA should therefore ideally present an input impedance equal to this value. Because the real part of the transducer impedance is small, ranging from 40 ohms to 400 ohms, the LNA should typically have a small input impedance.
An example of a circuit implementation of an LNA is illustrated in FIG. 3. LNA may be composed of an operational amplifier (op-amp) and two resistors. The purpose of the op-amp is to establish a virtual ground at the common terminal of the two resistors. The first resistor, Rf, converts an input voltage to a current and the second resistor, Rf, converts this current back to a voltage. The input impedance of the LNA circuit illustrated in FIG. 3 therefore is equal to Ri while the voltage gain is established by the ratio of Rf over Ri. The present invention relates to this type of LNA where the input voltage is first converted to a current.
Such a LNA typically is used only with low-voltage signals and is generally implemented in a silicon integrated circuit (IC) process that can not tolerate high-voltages. Therefore, it is necessary to position a transmit/receive (T/R) switch between the transducer and the LNA to prevent high-voltage pulses applied to the transducer, such as during the transmission time interval of an ultrasound system, from damaging the LNA. The T/R switch is open during the transmission time interval and closed during the reception time interval, thus passing only the low-voltage pulses to the LNA. An exemplary T/R switch is shown in FIG. 4A. This exemplary switch includes two Metal-Oxide-Silicon (MOS) devices. The gate-to-source voltage, defined as the potential difference between VG and VS, controls the resistance of the switch. The switch may thus be represented as a variable resistance as depicted in FIG. 4B. For a gate-to-source voltage below the threshold voltage of the MOS devices, the T/R switch presents a very high impedance and is therefore essentially open. The present invention disclosed herein, however, is not limited to the type of T/R switch.
Because the T/R switch is not ideal, i.e. demonstrating no resistance, it will exhibit a small resistance when closed. This resistance can be labeled ON resistance and denoted Ron. For purposes of illustration, the circuit in FIG. 5 shows a circuit comprising a T/R switch which is represented as an ideal switch in series with the ON resistance, and an op-amp-based LNA. The ON resistance of the T/R switch may be detrimental for at least two reasons. First, the ON resistance reduces the signal amplitude appearing at the input of the LNA because of a voltage divider effect. In particular, the signal amplitude at the input of the LNA will be the amplitude of the signal at the input terminal of the T/R switch scaled by the ratio of the input impedance of the LNA over the sum of the input impedance of the LNA and the ON resistance of the T/R switch. Second, the ON resistance generates electronic noise, which can reduce signal quality. The magnitudes of the adverse effects are proportional to the ON resistance.
Therefore, the ON resistance of a T/R switch must be reduced. In practice, the ON resistance preferably would be made much smaller than the LNA input impedance. Reducing the ON resistance of the T/R switch, however, would require increasing the size of the T/R switch circuit. Because larger circuits are more expensive to fabricate and also exhibit larger unwanted capacitances that limit high-frequency operation, an alternative approach is required.
There is thus a need for a circuit where the ON resistance of the T/R switch may be made larger, thus reducing fabrication costs and unwanted capacitances.